A Method for Predicting the Tendency of Oil Field Waters To Deposit Calcium Carbonate

Abstract

Abstract The authors previously presented a method for predicting the tendency of oilfield waters to deposit calcium sulfate. The present paper gives a similarmethod for calcium carbonate. Methods for predicting calcium carbonate scaling tendencies in fresh watershave been available for some time, but these could not be used for brines. Byexperimentally deriving the value of the K term in the Langelier equation, amethod has been developed which applies to waters of high salt content. Astatistical study is included which shows that the experimentally derivedvalues of K are in good agreement with actual conditions. Several applicationsof the final equation to production practice are given. Introduction The authors previously presented a method for predicting the formation ofcalcium sulfate scale in oil field waters. Although calcium sulfate depositionis important in production operations, the majority of scale problems involvecalcium carbonate. The present communication discusses a method by which theformation of this type of scale in oil field waters can be predicted. Calcium carbonate precipitation is caused by a shift toward carbonate in thecarbonate-bicarbonate-carbon dioxide equilibrium. When equilibrium shifts inthe other direction, the precipitate goes back into solution. Since there isusually considerable delay between the establishment of an equilibrium and theprecipitation or solution of calcium carbonate, unstable conditions exist inwhich a water will precipitate or dissolve calcium carbonate on standing. Tillmans, who did a major part of the early work on carbonate scaling, pointed out that the condition of equilibrium not only indicates the tendencyof a water to scale but also is an indication of its corrosive properties.Previously precipitated calcium carbonate combines with iron to form a densecrust which inhibits corrosion. If the water tends to dissolve carbonate, thescale becomes porous, and electrolytic corrosion takes place. Tillmans work wasextended by several investigators until in 1934 Langelier developed an equationsetting forth the conditions of carbonate equilibrium. By the use of thisequation the pH of a water at equilibrium can be calculated. If the actual pHis higher than the calculated pH, the water has a tendency to form scale. If itis lower, the water has a tendency to be corrosive. T.P. 3395